The Domain Name System (DNS) has a tree structure or hierarchy, with each non-RR (resource record) node on the tree being a domain name. A subdomain is a domain that is part of a larger domain; the only domain that is not also a subdomain is the root domain.
demo.cyotek.com is a subdomain of
sub.demo.cyotek.com is a subdomain of
demo.cyotek.com, however while
demo2.cyotek.com is a subdomain of
cyotek.com it is not a subdomain of